Survival and Metabolic Activity of Listeria monocytogenes on Ready‐to‐Eat Roast Beef Stored at 4 °C

Abstract

Three brands of commercial roast beef were purchased and artificially inoculated with a 5-strain Listeria monocytogenes cocktail at 2 inoculation levels (approximately 3 and 6 Log CFU/g). Although all 3 brands contained sodium diacetate and sodium lactate, inoculated Listeria cocktail survived for 16 d in all 3 brands; significant increases in L. monocytogenes numbers were seen on inoculated Brand B roast beef on days 12 and 16. Numbers of L. monocytogenes increased to 4.14 Log CFU/g for the 3 Log CFU/g inoculation level and increased to 7.99 Log CFU/g for the 6 Log CFU/g inoculation level by day 16, with the pH values being 5.4 and 5.8 respectively. To measure the cell viability in potential biofilms formed, an Alamar blue assay was conducted. Brand B meat homogenate had the highest metabolic activities (P < 0.05). By comparing its metabolic activities to Brands A and C and the inoculated autoclaved meat homogenates, results indicated that the microflora present in Brand B may be the reason for high metabolic activities. Based on the denaturing gradient gel electrophoresis and the Shannon-Wiener diversity index analysis, the "Brand" factor significantly impacted the diversity index (P = 0.012) and Brand B had the highest microflora diversity (Shannon index 1.636 ± 0.011). Based on this study, results showed that antimicrobials cannot completely inhibit the growth of L. monocytogenes in ready-to-eat roast beef. Native microflora (both diversity and abundance), together with product formula, pH, antimicrobial concentrations, and storage conditions may all impact the survival and growth of L. monocytogenes.

Listeria monocytogenes sensitivity to antimicrobial treatments depends on cell origin

“…Differences among the two matrices can be explained by the competition between spiked cells of L. monocytogenes and the microorganisms naturally present on foods. This aspect was already underlined by Broady et al 33 which observed the inability of L . monocytogenes to grow, even if inoculated, in some roast beef samples.…”

“…To mimic the worst scenario, these cells were used to contaminate 2 different food matrices. The attention was focused on Gorgonzola cheese and sliced roast beef, which can be susceptible to L. monocytogenes colonization 32 , 33 . Both matrices were inoculated with non-treated (control) and treated cells (55 °C and 55 °C TC).…”

Listeria monocytogenes Has Both Cytochrome bd -Type and Cytochrome aa 3 -Type Terminal Oxidases, Which Allow Growth at Different Oxygen Levels, and Both Are Important in Infection

“…It can grow within a wide temperature range and is resistant to low pH and high salt ( 4 ). This versatility may, at least in part, account for the ability of L. monocytogenes to survive food processing and to grow under conditions routinely used for food preservation ( 5 , 6 ).…”